1. Field of the Invention
The present invention relates to an amplifier apparatus that is used as a high-frequency amplifier apparatus for use in, for example, a wireless apparatus. More particularly, the invention concerns an amplifier apparatus that even when the ambient temperature changes can maintain the performance of sharply attenuating a frequency band adjoining to a frequency band serving as an object to be amplified.
2. Description of the Related Art
For example, a high-frequency amplifier apparatus that is used in a conventional wireless apparatus is constructed in multiple stages. As a method of limiting the pass frequency band, a band pass filter is connected to an input or output of the high-frequency amplifier apparatus. Or, a bandpass filter is connected to between two adjoining stages of the high-frequency amplifier apparatus. By doing so, there is generally made up a construction of limiting the frequency pass band and thereby attenuating a frequency band the amplification of that is unnecessary.
In FIG. 2, there is illustrated a construction of the above-described conventional high-frequency amplifier apparatus. A reference numeral 1 denotes a band pass filter that causes a signal having a frequency band (a peculiar-to-apparatus band, i.e., an original band), an object to be amplified, to be passed therethrough and that causes the other frequency bands to be attenuated down to a required level. A reference numeral 2 denotes a low-noise amplifier that low-noise-amplifies the original band signal that has passed through the band pass filter 1. A reference numeral 3 denotes a power amplifier that amplifies the low-noise-amplified signal up to a required high-frequency power. A reference numeral 4 denotes a bandpass filter that attenuates an unneeded frequency band.
In the above-described high-frequency amplifier apparatus, it is effective in case its required performance is seen in the circumstances where the attenuation characteristics of frequencies other than the pass band frequency are gentle and the attenuation at an adjoining frequency band is relatively small. In FIG. 3 there is illustrated a filter transmission characteristic. Assume that the characteristic indicated surrounded by each mark A in the figure be the required filter transmission characteristic. Then, a characteristic 5 shows that a transmission characteristic further sharpening the gradient of the filter transmission characteristic is required, while a characteristic 6 shows that the gradient of the transmission characteristic meets the required gradient. However, in the latter case, because the characteristic 6 is made to be the one wherein the pass bandwidth B is narrowed, a transmission characteristic making the pass bandwidth wider is required.
In order to realize the filter transmission characteristic meeting the above-described required transmission characteristic, it becomes necessary to make up the following construction. Namely, wherein the gradient of the transmission characteristic is made sharp by increasing the number of the filter stages; or the transmission loss is made small and the end of the pass band is made acute by enlarging the volume of the filter per se.
However, this construction is inevitably followed by an increase in the volume and weight of the high-frequency amplifier apparatus, as well as by an increase in the cost that occurs due to the increase in the cost for adjustment.
On this account, as a high-frequency amplifier apparatus that can solve the increase in the stages number and volume of the pass band filter, which are the above-described points in problem, to thereby enable miniaturization and economization of it, the following construction has hitherto been proposed. Namely, wherein an original signal is converted by a frequency converter to a frequency falling within an intermediate-frequency band (IF band) the frequency of that is lower than the original frequency. Then, the IF band is connected to a filter of a required transmission characteristic. Then, this IF band is again converted to the original frequency by a frequency converter.
In FIG. 4 there is illustrated the construction of a high-frequency amplifier apparatus in which to convert the signal into the IF band for processing thereof. A reference numeral 11 denotes a band pass filter that permits the passage therethrough of a signal having a frequency band (an original band: fr) to be amplified. A reference numeral 12 denotes a low-noise amplifier that low-noise-amplifies the signal the frequency of that is the original band. A reference numeral 13 denotes a frequency converter that converts the signal having the original band to an IF band (fi). A reference numeral 14 denotes an IF frequency band pass filter that permits the passage therethrough of the signal having the IF frequency band (fi). A reference numeral 15 denotes an IF frequency band amplifier that amplifies the signal having the IF band. A reference numeral 16 denotes an IF frequency band pass filter that permits the passage therethrough of the signal having the IF frequency band (fi). A reference numeral 17 denotes a frequency converter that converts the signal having the IF frequency band (fr) to the original band (fr). A reference numeral 18 denotes a power amplifier that amplifies the converted signal having the original band up to a high-frequency power. And a reference numeral 19 denotes a band pass filter that attenuates an unneeded frequency band.
And, in this high-frequency amplifier apparatus, an oscillation frequency signal from a local oscillator 20 is distributed by a distributor 21 into the converters 13, 17. And, by multiplying the original band by a local oscillation frequency, frequency conversion is performed between the original band and the IF band.
Here, in FIG. 5, there are illustrated a transmission characteristic 22 of a filter the center frequency of that is a frequency fr0 and a transmission characteristic 23 of a filter the center frequency of that is a frequency fi0 lower than the frequency fr0. As illustrated in the figure, assuming that the pass bandwidth xcex94f be the same in the bandwidth in terms of either filter, it is known that the following can be said. Namely, in case the number of stages is the same, the transmission characteristic that is lower in the center frequency becomes sharper in gradient of attenuation. Namely, in FIG. 5, the following is known in terms of the relationship that when the frequency displacements xcex94f from the both center frequencies are the same in value holds true between the amounts xcex11, xcex12 of attenuation of the respective filters. Namely, the xcex11 is greater than the xcex12.
Accordingly, in the case of constructing a multi-stage amplifier apparatus for limiting the pass band of frequency, when making great the amount of attenuation of an adjoining frequency, there has hitherto been used the amplifier apparatus such as in FIG. 4. Namely, the amplifier apparatus for converting the original frequency to the IF band.
However, in case the amplifier apparatus has been made to have such a construction as illustrated in FIG. 4, the band pass filter 14 or 16 for passing the IF frequency band has a center frequency of its pass band shifted due to a change in the ambient temperature. Resultantly, a shift of the frequency occurs in the transmission characteristic of the filter, with the result that the filter has the inherent drawback incapable of filtering its required band of frequency.
In FIG. 6 there is illustrated a change in the transmission characteristic due to a change in the ambient temperature of the band pass filter. The characteristic 24 indicated by a solid line represents the filter transmission characteristic at normal temperature while the characteristic 25 indicated by a broken line represents the filter transmission characteristic at the time of a change in the temperature.
In case having used the above-described filter whose characteristic varies depending upon the temperature, the characteristic of the high-frequency amplifier apparatus illustrated in FIG. 4 is approximately determined according to the transmission characteristics of the band pass filters 14, 16 each for the IF band. Namely, that characteristic inconveniently becomes dependent upon the ambient temperature very much. Especially, in a frequency range in which the attenuation characteristic is sharp, the transmission characteristic of the amplifier apparatus becomes deteriorated due to the change in the temperature. As a result, there was the problem that it was impossible to realize a stable transmission characteristic of the amplifier apparatus.
The present invention has been made in view of the above-described conventional circumstances and has an object to provide an amplifier apparatus arranged to select the pass band, which has corrected the shift of the filter transmission characteristic due to a change in the temperature and can stably amplify a high-frequency signal, and can be made small in size and low in cost.
Another object of the present invention is to provide an amplifier apparatus in which correcting the shift of the filter transmission characteristic as above can be arbitrarily adjusted.
To attain the above object, the present invention provides an amplifier apparatus which is adapted to convert a signal to be processed into one having an intermediate frequency band to thereby execute amplification processing and which includes a local oscillator the oscillation frequency of that can be altered, a first frequency converter that converts the signal to be processed by the use of the oscillation frequency from the local oscillator into an intermediate frequency, a first intermediate frequency band/band pass filter that limits to an intermediate frequency band the signal to be processed having the intermediate frequency that has been output from the first frequency converter, an intermediate-frequency amplifier that amplifies the signal to be processed having the intermediate frequency that has passed through the first intermediate frequency band/band pass filter, a second intermediate frequency band/band pass filter that limits to an intermediate frequency band the signal to be processed having the intermediate frequency that has been output from the intermediate-frequency amplifier, a second frequency converter that converts the signal to be processed having the intermediate frequency that has passed through the second intermediate frequency band/band pass filter into the original frequency by the use of the oscillation frequency from the local oscillator, a temperature sensor that detects the temperature of each of the first and second intermediate frequency band/band pass filters, and a controller that alters the oscillation frequency of the local oscillator in correspondence with the detected temperature and thereby stabilizes the transmission characteristic of each of the first and second intermediate frequency band/band pass filters irrespective of the change in the temperature.
Here, it may be arranged that the temperature detector directly detects the temperature of each of the first and second intermediate frequency band/band pass filters. Or it may be arranged that by detecting the temperature of the ambient zone, in which each of the first and second intermediate frequency band/band pass filters has been installed, the temperature of each of these filters is indirectly detected.
Also, the amplifier apparatus according to the present invention has equipped thereto a memory for having stored therein the frequency shift data of the transmission characteristic of each of the first and second intermediate frequency band/band pass filters due to the change in the temperature. And the controller alters the oscillation frequency of the local oscillator according to the frequency shift data read out from the memory and the detected temperature obtained from the detector. Thereby, the amplifier apparatus can even cope with another intermediate frequency band/band pass filter, the temperature characteristic of that is different, by reloading the shift data of the memory or replacing the memory with another memory having stored therein different shift data.